Surprise! The first peek inside Mars reveals a crust with pie-like layers

The Mars InSight lander is equipped with a dome seismometer (left) and a probe to measure heat flux (right).Credit: NASA / JPL-Caltech

NASA’s InSight mission finally peeked inside Mars and found that the planet’s crust could be made up of three layers. This is the first time that scientists have directly explored the interior of a planet other than Earth and will help researchers unravel how Mars formed and evolved over time.

Prior to this mission, researchers had only measured the internal structures of the Earth and the Moon. “This information was missing, until now, from Mars,” said Brigitte Knapmeyer-Endrun, a seismologist at the University of Cologne in Germany, in a pre-recorded speech given at the virtual meeting of the American Geophysical Union on December 15. He declined an interview with Nature, stating that the work is being studied for publication in a peer-reviewed journal.

It is a major discovery for InSight, which landed on Mars in November 2018 with the aim of elaborating the internal structure of the planet.1. The InSight lander crouches near the Martian equator, on a smooth plain known as Elysium Planitia, and uses an exquisitely sensitive seismometer to listen to the geological energy pulsing through the planet2. So far, the mission has detected more than 480 “marsquakes,” says Bruce Banerdt, principal investigator of the mission and scientist at the Jet Propulsion Laboratory in Pasadena, California. Mars is less seismically active than Earth, but more than the Moon.

Just as they do with earthquakes on Earth, seismologists are using earthquakes to map the internal structure of the red planet. Seismic energy travels through the ground in two types of waves; By measuring the differences in the way these waves move, researchers can calculate where the planet’s core, mantle and crust begin and end, and the overall composition of each. These fundamental geological layers reveal how the planet cooled and formed billions of years ago at the fiery birth of the Solar System. Now, “we have enough data to start answering some of these big questions,” Banerdt says.

The Earth’s continental crust is generally divided into sub-levels of different types of rock. The researchers suspected, but didn’t know for sure, if the Martian crust was also layered, says Justin Filiberto, a planetary geologist at the Lunar and Planetary Institute in Houston, Texas. Now, the InSight data shows it is made up of two or three layers.

A three-layered crust would fit better with geochemical models3 and studies on Martian meteorites, says Julia Semprich, a planetary scientist at the Open University in Milton Keynes, UK.

Depending on whether the crust actually has two or three layers, it is 20 or 37 kilometers thick, Knapmeyer-Endrun said during his speech. The thickness likely varies in different locations on the planet, but it is likely not to exceed 70 kilometers on average, he added. On Earth, the thickness of the crust varies from about 5 to 10 kilometers under the oceans, to about 40-50 kilometers under continents.

In the coming months, InSight scientists plan to report measurements taken even deeper on Mars, Banerdt says, eventually revealing information about the planet’s core and mantle.

Along with listening to earthquakes, InSight’s other big scientific goal is to measure the flow of heat across the Martian soil using a probe dubbed the mole. He had to bury himself deep in the ground, but he struggled to do it – at one point he even jumped out of the ground. The mole has finally managed to reach a depth of several inches, Banerdt says, and in the coming weeks it will try to dig one last time before giving up. “We are at what we consider the final game now,” he says.